The polymerization of various olefins, including propylene, ethylene, and the like has been known in the chemical art for quite some time. Generally speaking, in order to polymerize an olefin, one provides the olefin to be polymerized (the “monomer”) and contacts the olefin monomer with a catalytic system under sufficient conditions of temperature, pressure, and composition to cause polymerization of the monomer. The conditions of temperature and pressure may be varied, as well as the type of reaction vessel in which the polymerization is carried out.
One process for polymerization of olefins including, but not limited to propylene is known as the slurry process. In the slurry process, an inert organic solvent is fed into a closed reaction vessel and typically heated, with stirring. Then, a monomeric raw material is fed into the reaction vessel wherein some of the monomer dissolves in the solvent. Catalyst is fed to the stirred reactor and the monomer becomes polymerized. Polymer and solvent may be removed as a slurry, provided that the polymer, by its very nature, has no tendency to stick to the reactor walls, through a pipe in one of the sides or bottom of the reactor. The polymer is then separated by the solvent using means well known to those skilled in the polymer art, and the solvent is recycled. The process may be conducted as a batch process, and the monomer itself may function as the solvent, as in the case when propylene is employed under conditions in which it exists in the liquid state. The slurry process is well-known in the art.
Another process for polymerization of olefins including, but not limited to propylene is known as the gas-phase process. The gas-phase reaction of olefin monomers to form polyolefins is generally conducted in a fluidized bed in the presence of a suitable heterogeneous catalyst. The polymer is then removed from the reactor and further processed using means well known to those skilled in the polymer art. The gas phase or fluidized bed process is most typically conducted in continuous fashion.
Polyolefins are commercially important for their use in diverse products due to the unique combination of chemical and physical properties they may be caused to possess, including, inter alia: chemical inertness, softness, flexibility, and recyclability. These and other of the various properties of polyolefins may be altered, as is known to those skilled in the art, by changing such process variables as catalyst system composition, catalyst concentration, co-catalyst composition, co-catalyst concentration, monomer concentration, monomer composition, temperature of reaction, and hydrogen pressure in the reactor. Since there are so many potential process variables associated with the production of polyolefins, the number of possible combinations of the aforesaid coupled with the large number of chemical compounds available to function as catalyst and co-catalyst have caused the polyolefin chemical arts to become a crowded field of art.
While the physical and mechanical properties of polymers are widely changeable depending upon manufacturing variables, there nevertheless exists a general set of measurements which are commonly used by those in the polymer arts for classifying polymers. Some of the more common measurements and physical properties are: average molecular weight, molecular weight distribution or polydispersity index (“PDI”), MEK soluble fraction, xylene soluble fraction, heptane soluble fraction, Shore D hardness; tensile modulus, tensile stress, melt swell ratio, EP rubber content in the case of ethylene/propylene copolymers, melt flow rate, melt viscosity, VICAT softening point, crystallinity, isotactic pentad content, syndiotactic pentad content, etc.
A number of patents disclose catalysts and processes to prepare non-conventional polyolefins, including U.S. Pat. Nos. 4,524,195; 4,736,002; 4,971,936; 4,335,225; 5,118,768; 5,247,032; 5,565,532; 5,608,018; and 5,594,080, as well as European Patents EP 604908 and 693506, the entire contents of all aforesaid patents being herein incorporated by reference thereto.
Compounded thermoplastic olefin compositions (TPOs) are defined as blends of polypropylene, olefinic elastomers and optionally fillers and other compounding ingredients. TPOs are recognized in the art as being multiphase polymer blends where a polypropylene homopolymer forms a continuous matrix phase and the elastomer and fillers are the dispersed components. The polypropylene homopolymer matrix imparts tensile strength and chemical resistance to the TPO while the elastomer imparts flexibility and impact resistance. Traditionally, ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) have been used as the elastomeric component in TPOs. Recently, other ethylene-alpha olefin copolymers have been used, especially ethylene-butene and ethylene-octene copolymers. Typically, a polypropylene-based TPO material is composed of a high ethylene content polypropylene copolymer resin and the post-reactor addition of a EPDM, EPM, SEBS, EOM, or other suitable rubber to give the final product higher impact properties. Compounding the material post-reactor involves another step and the rubber cannot be dispersed into the polymer matrix at a molecular level as well. The size of the rubber particles are also much larger than those which can be made in the reactor. Chemicals such as peroxide which increase the melt flow rate of the final product are also added, post-reactor.
One major market for TPOs is in the manufacture of automotive parts, especially bumper fascia and other energy-management parts such as pillars. These parts are generally made using injection molding processes. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate>35). However, these high melt flow rate (MFR) resins are difficult to toughen, resulting in products that have low impact strength.
One of the reasons impact modification by compounding of high MFR polypropylene resins is difficult is because of the large differences in the melt viscosities between the polypropylene resins employed and the elastomers typically used as impact modifiers. These differences lead to a poor dispersion of the elastomer in the polypropylene matrix, resulting in large dispersed elastomer particle sizes which, in turn, is detrimental to overall impact strength.
One proposed solution to the problem has been to decrease the molecular weight of the elastomer used, in order to reduce the viscosity of the elastomer. While this route produces better dispersion of the elastomer in the polypropylene matrix, the reduced molecular weight of the modifier adversely affects the impact strength of the TPO.
Another proposed solution has been to develop products which behave like a low viscosity plastic during the mixing process, yet function like an elastomer in the molded TPO. These type of polymers are generally referred to as plastomers. To date, however, these plastomer products have not yielded satisfactory impact performance when used with high melt flow polypropylene.
A third area that has been explored is the use of branched elastomers. U.S. Pat. No. 5,681,897 discloses the use of substantially linear ethylene-alpha olefin copolymers having a degree of long chain branching as impact modifiers for polypropylene as well as other thermoplastic resins. While the use of these elastomers appears to lead to an improvement in impact strength, there is still a need for impact strength and stiffness in TPOs made with high MFR polypropylene resins.
However, although many workers have tried for decades to provide reactor-grade polymers suitable for employment in applications where TPO's are called for, none have thus far succeeded in providing a thermoplastic polyolefin which simultaneously exhibits excellent impact properties at low temperatures and is possessed of a substantial degree of stiffness. Further, no worker(s) has succeeded in providing such a reactor-grade polymer which additionally has a relatively high melt flow rate (“MFR”), and low xylene-soluble fraction.
By our invention, we have achieved production of such polymers as they emerge from the reactor. For this reason, the polymers of this invention may be referred to as “reactor grade” polymers. The improvement in stiffness of our reactor grade polymers over those available in the prior art will allow a reduction in the level of fillers that are otherwise normally needed to increase the stiffness and flexural modulus of the polymers. This in turn will allow thinner articles to be made which may have been limited in the past due to the melt flow of similar materials being too low or not cost competitive due to post addition of modifiers and the costs associated therewith. The benefit of a high melt flow rate along with the other advantageous properties possessed by the polymers of the present invention will allow the polymers provided herein to be used in thinner-walled and more intricately designed products. A broader molecular weight distribution of the polymers of the present invention gives these polymers better impact to stiffness balance and makes these materials more shear-sensitive, which in turn allows our materials to flow easier under conditions of high shear that are typically experienced in conventional molding practices.